Electro-optical detector protection device

ABSTRACT

A protection device for an electro-optical detector used in systems for making far field diagnostic measurements of a high energy laser beam comprises a solid silicon cone having a truncated apex which functions as the pin hole aperture for the detector. The large end of the cone has a surface defined by the radius drawn from the cone axis at the truncation. This surface is coated with a reflective attenuation layer which reflects a high percentage of the energy of the incident laser beam entering the pinhole aperture thereby minimizing the absorption of the beam by the cone. The high index of refraction of the solid silicon cone permits large off-axis angles of the laser beam to be directed onto the detector resultingly increasing the field of view of a system using a given detector. The increased field of view allows a closer spacing of a plurality of detectors in the system ensuring a high spatial resolution of the laser beam for a desired field of view.

BACKGROUND OF THE INVENTION

This invention relates to a field of optical apparatus and more particularly to apparatus for protecting an electro-optical detector used in a system for field diagnostic measurements of a continuous wave high energy laser beam.

Electro-optical detectors are generally operated in conjunction with primary optical systems for gathering and focusing the reflected radiation from a desired "target" or source, toward which the optical axis of the system is accordingly directed. In far field diagnostic measurements, a far field target is irradiated with a continuous high energy laser beam with a beam "on" time from 3-30 seconds. The intensity pattern in the far field is thereafter mapped using electro-optical detectors. However, the intensity of the target radiation is generally of a magnitude which, if unattenuated, can lead to detector failure or burnout.

SUMMARY OF THE INVENTION

Accordingly, one object of this invention is the provision of an apparatus for protecting electro-optical detectors from the high power irradiance of a laser beam.

Another object is the provision of an electro-optical detector protection device for use in far field diagnostic measurements of a high energy laser beam which allows large off-axis angles of the laser beam to be directed on a relatively small detector.

Briefly, these and other objects are attained by positioning a solid pin hole lens upstream of an electro-optical detector which is used in the far field diagnostic laser measurement. A silicon cone, having a truncated apex, acts as the pin hole aperture, and has a reflective surface defined by a radius drawn from the cone axis at the truncation. This surface has a reflective attenuator coating disposed thereon which serves to reflect most of the laser energy back out through the aperture thereby minimizing the absorption of the beam by the solid pinhole cone. The high refractive index of the solid silicon cone permits large off axis angles of an incident laser beam to be directed onto a detector. This factor increases the field of view of the electro-optical detection system for a given detector thereby obtaining high spatial resolution of the laser beam. A plurality of target detectors can be employed to cover the desired field of view.

BRIEF DESCRIPTION OF THE DRAWING

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the sole accompanying drawing wherein an optical schematic of the electro-optical detector protection device is given.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing wherein a plurality of detector protection devices is shown positioned in a far field diagnostic measurement device housing 10. Fixed within housing 10 are a plurality of identical solid cones 12 disposed in a suitable array. Each cone 12 is truncated at its apex forming a pin hole aperture 14 and is constructed out of a material having a high index of refraction such as silicon. The large end of the cone has a surface 16 whose radius of curvature is centered at the truncation or at aperture 14. This surface is coated with a reflective attenuation coating, which in combination with the curvature of 16, ensures that most of the incident laser energy which enters through aperture 14 is reflected back out through the aperture. This arrangement thus minimizes the absorption of a laser beam a-b by the solid pin hole cone 12.

Contiguous to each cone 12 is a spacer 18 fixed within housing 10 which allows a field lens 20 to direct the laser beam through an attenuator such as lens 22 to a target unit or detector 24. The attenuator serves to absorb laser energy in order to further protect an associated detector. The target unit 24 can obviously be of many types of electro-optical devices but preferably may be a pyroelectric detector, each having its own protection device.

A plurality of target units may be employed in a suitable array to cover the desired field of view. The pyroelectric detectors could be sampled by switching from one detector to another using any conventional control system. The use of a pyroelectric detector for monitoring the laser beam irradiance is relatively conventional with each detector signal being sampled by an FET switch and commutated into a single preamp.

The present device not only reduces the possibility of detector destruction due to the incidence and measurement of a high energy laser beam such as a continuous wave laser beam but allows a relatively large off-axis angle of the incident laser beam to be directed onto a relatively small detector 24. This is accomplished by using the solid truncated cone having a high refractive index, as for example silicon.

As shown in the sole figure, an incident beam a-b enters pinhole aperture 14 at a large oblique angle to electro-optical detector and protection device axis x-y. After the beam a-b is refracted by the cone 12, most of the laser beam is reflected back out through aperture 14 as a'-b'. Some of the laser beam, is attenuated by the coating on surface 16 whereafter it, i.e., a"-b", freely passes through spacer 18. Field lens 20 further refracts the laser beam and directs the remittant beam a"'-b"' through attenuator 22 and onto the target detector 24. This arrangement prevents "cross-talk" between detectors so that a laser beam entering a particular pinhole aperture will arrive at its associated detector and not another random detector. Furthermore, since large off-axis angles of an incident laser beam can be directed onto a relatively small detector, the field of view of the system is increased for a given size detector. As a result, the detectors can be spaced physically closer together thereby achieving a higher spatial resolution of the laser beam.

Thus, what has been described is a electro-optical detector protection device utilizable in far field diagnostic measurements of a high energy laser beam which precludes destruction of the detector while allowing large-off axis angles of an incident laser beam to be directed onto a relatively small detector thereby increasing the field of view and spatial resolution of the system for a given sized detector.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be expressly understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. 

What is claimed is:
 1. An improved system for making far field diagnostic measurements of an incident laser beam comprising:a plurality of electro-optical detectors arranged in a suitable array; a plurality of electro-optical detector protection means, one protection means being associated with each detector:each protection means comprising means capable of optically transmitting and refracting said laser beam, said last mentioned means having means for reflecting most of the energy of said laser beam away from its associated detector and allowing a reduced energy level laser beam to be directed at its associated detector; means for focusing said reduced energy level laser beam on to its associated detector; and means disposed between said focusing means and the associated detector for further attenuating said reduced energy level laser beam before it impinges upon the associated detector.
 2. An electro-optical detector protection device for use in a system for making far field diagnostic measurements of an incident high energy beam of light comprising:a solid cone capable of optically transmitting and refracting said incident beam of light, said cone having a truncated apex acting as a pinhole aperture, a surface of said cone having means disposed thereon for reflecting most of the energy of said incident beam of light back out through said aperture; means for directing the remainder of said incident beam of light onto an electro-optical detector; and means disposed between said last mentioned means and said detector for attenuating said incident beam of light before it impinges on said detector.
 3. The device of claim 1 wherein said surface is defined on the large end of said cone by a radius drawn from the axis of said truncated cone at said pinhole aperture.
 4. The device of claim 3 wherein said means disposed on said surface for reflecting most of the energy of said incident beam of light back through said aperture is a reflective attenuation coating.
 5. The device of claim 4 wherein said means for directing the remainder of said incident beam of light comprises a field lens whose axis is coincident with the axis of said cone; anda spacer juxtaposed between said field lens and said surface, said spacer being contiguous to said surface.
 6. The device of claim 5 wherein said attenuating means comprises an attenuating lens.
 7. An improved system for making far field diagnostic measurements of an incident laser beam comprising:a plurality of electro-optical detectors arranged in a suitable array; a plurality of electro-optical detector protection means, one protection means being associated with each detector: each protection means comprising means capable of optically transmitting and refracting said laser beam, said last mentioned means having means for reflecting most of the energy of said laser beam away from its associated detector and allowing a reduced energy level laser beam to be directed at its associated detector; means for focusing said reduced energy level laser beam on to its associated detector; and means disposed between said focusing means and the associated detector for further attenuating said reduced energy level laser beam before it impinges upon the associated detector; wherein said means capable of optically transmitting and refracting said laser beam comprises a solid cone having a truncated apex acting as a pinhole aperture, said cone having a surface for directing most of the energy of said incident laser beam back out through said aperture.
 8. The system of claim 7 wherein said surface is defined on the large end of said cone by a radius drawn at the longitudinal axis of said cone from said aperture.
 9. The system of claim 7 wherein the material of said cone is silicon.
 10. The system of claim 7 wherein said surface has a reflection attenuating coating disposed thereon.
 11. The system of claim 8 wherein said focusing means comprises a field lens whose axis is coincident with the axis of said cone; anda spacer juxtaposed between said field lens and said surface, said spacer being contiguous to said surface.
 12. The system of claim 11 wherein said attenuating means comprises an attenuating lens. 